This application is a continuation-in-part of Snyder et al., U.S. Ser. No. 149,573 entitled xe2x80x9cCarbon Fibrilsxe2x80x9d filed Jan. 28, 1988, now abandoned which is a continuation-in-part of Tennent et al., U.S. Ser. No. 872,215 entitled Novel Carbon Fibrils, Method for Producing Same and Compositions Containing Same, filed Jun. 6, 1986, now abandoned, which is a continuation-in-part of Tennent, U.S. Ser. No. 678,701 entitled Carbon Fibrils, Method for Producing Same and Compositions Containing Same, filed Dec. 6, 1984, now U.S. Pat. No. 4,663,230 which are assigned to the same assignee as the present application and hereby incorporated by reference in their entirety.
This invention relates to elastomeric composites.
Elastomers have been filled with a variety of materials. Such materials are used to improve the mechanical or electrical properties of the elastomer matrix, or to reduce cost.
Carbon fibrils are carbon filaments having diameters less than 500 nanometers. Examples of particular carbon fibrils and methods for preparing them are described in the above-referenced Snyder et al. and Tennent et al. applications; and the Tennent patent as well as in Tennent et al., U.S. Ser. No. 871,676 filed Jun. 6, 1986 (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Compositions Containing Samexe2x80x9d); Tennent et al., U.S. Ser. No. 871,675 filed Jun. 6, 1986 (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Encapsulated Catalystxe2x80x9d); Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 (xe2x80x9cFibrilsxe2x80x9d); and McCarthy et al., U.S. Ser. No. 351,967 filed May 15, 1989 (xe2x80x9cSurface Treatment of Carbon Microfibersxe2x80x9d), all of which are assigned to the same assignee as the present application and are hereby incorporated by reference in their entirety.
In general, the invention features a composite in which carbon fibrils are incorporated in an elastomer matrix. In one aspect, the fibrils are characterized as having a morphology consisting of tubes that are free of a continuous thermal carbon overcoat (i.e. pyrolytically deposited carbon resulting from thermal cracking of the gas feed used to prepare the fibrils) and have graphitic layers that are substantially parallel to the fibril axis. One aspect of substantial parallelism is that the projection of the graphite layers on the fibril axis extends for a relatively long distance in terms of the external diameter of the fibril (e.g., at least two fibril diameters, preferably at least five diameters), as described in Snyder et al., U.S. Ser. No. 149,573. These fibrils preferably have diameters less than 100 nanometers, more preferably between 3.5 and 75 nanometers, inclusive, and a length to diameter ratio of between 5 and 100.
In a second aspect, the fibrils are characterized as having a crystalline graphitic structure and a morphology defined by a fishbone-like arrangement of the graphite layers along the fibril axis. Examples of such fibrils are described in the aforementioned Snyder et al., application, U.S. Ser. No. 149,573 and in Geus et al., European Application No. 0 198 558 published Oct. 22, 1986. These fibrils preferably have diameters less than 100 nanometers.
The amount of fibrils in the composite is preferably sufficiently high to permit curing of the composite by resistive or inductive heating or to permit at least one of the physical properties of the composite to be monitored electrically; preferably, this amount is less than 25 parts per 100 parts of elastomer, more preferably less than 10 parts per 100 parts of elastomer. In the case of masterbatches (i.e. fibril-filled elastomer precursors which are subsequently blended with additional elastomer in order to prepare the final composite structures), however, the amount of fibrils is preferably greater than 25 parts per 100 parts of elastomer.
Preferred elastomer matrices include natural rubber, styrene-butadiene rubber (both random and block copolymers), polyisoprene, neoprene, chloroprene, polybutadiene (both cis and trans 1,4 and 1,2-polybutadienes), fluoroelastomers (e.g., fluorinated polyethylene), silicone rubbers, and urethane elastomers. In addition to the fibrils, the elastomer preferably contains one or more fillers, e.g., carbon black, silica, or a combination thereof; the ratio of the amount of fibrils in the composite to the total amount of the fillers is at least 1:4 or better (e.g., 1:5, 1:6, etc.). The composites are preferably provided in the form of a tire or component thereof (e.g., tire tread or casing), seal, solution, or adhesive.
In a third aspect, the invention features a method for curing an elastomer that includes the steps of preparing a composite by incorporating carbon fibrils in an elastomer matrix, the amount of the fibrils being sufficient to impart to the composite an electrical conductivity sufficiently high to permit resistive or inductive heating, and heating the composite resistively or inductively to effect cure.
In a fourth aspect, the invention features a method for monitoring the physical condition of an elastomer that includes the steps of preparing a composite by incorporating an electrically conductive additive in an elastomer matrix, the amount of the additive being sufficient to impart to the composite an electrical conductivity sufficiently high to permit the physical condition of the elastomer to be monitored electrically, and monitoring the electrical properties (e.g., resistivity) of the composite as an indication of the physical condition of the elastomer. In a preferred embodiment of this aspect, the electrically conductive additive includes carbon fibrils. In another preferred embodiment, the composite is in the form of a tire and the the pressure inside the tire is monitored. The method is also preferably used to monitor an elastomer (e.g., in the form of a conveyor belt or hose) for the presence of rips, tears, or perforations.
In preferred embodiments of the third and fourth aspects, the amount of fibrils in the composite is less than 25 parts per 100 parts elastomer, more preferably less than 10 parts per 100 parts elastomer. Preferred fibrils are those described above.
In a fifth aspect, the invention features a method for preparing an elastomer composite that includes the steps of preparing a masterbatch by dispersing in an elastomer at least 25 parts of fibrils per 100 parts of elastomer, and compounding a predetermined portion of the masterbatch with an additional amount of an elastomer which may be the same as or different from the elastomer used to prepare the masterbatch) to prepare the composite. Preferably, the amount of fibrils in the final composite is less than 25 parts per 100 parts elastomer, more preferably less than 10 parts, Preferred fibrils are as described above. Carbon black may also be added to the composite, either during preparation of the masterbatch or during the compounding step.
In a sixth aspect, the invention features a method for reinforcing an elastomer that includes incorporating into an elastomer matrix an amount of carbon fibrils sufficient to improve the mechanical properties of the elastomer. The fibrils are as described above for the first and second aspects of the invention.
The invention provides fibril-reinforced elastomer composites exhibiting good stiffness, tensile strength, tear strength, creep and die swell resistance, and green strength (i.e. strength prior to cure). The composites also exhibit good hardness, stress-strain properties, and abrasion resistance (even with relatively soft elastomer matrices), and have low specific gravity. The improved abrasion resistance makes it possible to achieve an advantageous balance of traction, rolling resistance, and tread wear in articles fabricated from the composites. Moreover, these advantages are achieved at low fibril loadings.
Further advantages result from the electrical properties of the fibrils. Because the fibrils are electrically conductive, they can be used to perform the dual functions of reinforcing an elastomer matrix and at the same time rendering the matrix electrically conductive. The electrically conductive composites can be cured by resistive or inductive heating, thus avoiding the problems of thermal transfer and high cost often associated with conventional heat curing. The ability to be cured electrically makes the fibril-filled composites particularly useful as adhesives in a variety of bonding operations such as forming rubber-rubber bonds (e.g., in bonding tire treads to tire casings), rubber-metal bonds, and rubber-ceramic bonds, and in rubber repair systems for products such as tires and conveyor belts.
The electrical conductivity of the composites is also useful in applications such as the extrusion of rubber tubes, treads, and related products. In addition, the composites can be partially electrically cured to impart additional dimensional stability to the composite. The invention also makes possible the design products in which physical changes in the composite such as internal pressure can be monitored electrically. For example, the state of a tire, air spring, hose, or conveyor belt can be monitored simply and effectively.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.